A colony of pure bred dogs with an apparent heart conduction defect has been establshed. This spontaneous model has a tendency toward syncopal attack and sudden death. Continuous monitoring of the electrocardiogram of awake unrestrained dogs by radio telemetry suggests that period of sinus asystole are associated with the attacks. Monitoring of additional variables (blood pressure, respiration, electroencephalogram and electrogram) is being initiated to further characterize the syncopal attacks and conditions associated with the attacks. Pacing studies are being used to further characterize the conduction defect and to diagnose susceptible individuals. Pharmacologic studies are being used to aid in characterizing the attacks and possible prevention.